It is known that antenna performance is dependent on the size, shape and material composition of the antenna elements, the interaction between elements and the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These physical and electrical characteristics determine several antenna operational parameters, including input impedance, gain, directivity, signal polarization, resonant frequency, bandwidth and radiation pattern. Since the antenna is an integral element of a signal receive and transmit path of a communications device, antenna performance directly affects device performance.
Generally, an operable antenna should have a minimum physical antenna dimension on the order of a half wavelength (or a multiple thereof) of the operating frequency to limit energy dissipated in resistive losses and maximize transmitted or received energy. Due to the effect of a ground plane image, a quarter wavelength antenna (or odd integer multiples thereof) operative above a ground plane exhibits properties similar to a half wavelength antenna. Communications device product designers prefer an efficient antenna that is capable of wide bandwidth and/or multiple frequency band operation, electrically matched (e.g., impedance matching) to the transmitting and receiving components of the communications system, and operable in multiple modes (e.g., selectable signal polarizations and selectable radiation patterns).
Given the advantageous performance of quarter and half wavelength antennas, conventional antennas are typically constructed so that the antenna length is on the order of a quarter wavelength of the radiating frequency and the antenna is operated over a ground plane, or the antenna length is a half wavelength without employing a ground plane. These dimensions allow the antenna to be easily excited and operated at or near a resonant frequency (where the resonant frequency (f) is determined according to the equation c=λf, where c is the speed of light and λ is the wavelength of the electromagnetic radiation).
Half and quarter wavelength antennas limit energy dissipated in resistive losses and maximize the transmitted energy. But as the operational frequency increases/decreases, the operational wavelength decreases/increases and the antenna element dimensions proportionally decrease/increase. In particular, as the frequency of the received or transmitted signal decreases, the dimensions of the quarter wavelength and half wavelength antenna proportionally increase to maintain a resonant condition. The resulting larger antenna, even at a quarter wavelength, may not be suitable for use with certain communications devices, especially portable and personal communications devices intended to be carried by a user. Since these antennas tend to be larger than the communications device with which they operate, the antenna is typically mounted with a portion of the antenna protruding from the communications device. Such mounting schemes subject the antenna to possible damage.
The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less obtrusive, and more efficient antennas that are capable of wide bandwidth or multiple frequency-band operation, and/or operation in multiple modes (i.e., selectable radiation patterns or selectable signal polarizations). For example, operation in multiple frequency bands may be required for operation of the communications device with multiple communications systems or signal protocols within different frequency bands. For example, a cellular telephone system transmitter/receiver and a global positioning system receiver operate in different frequency bands using different signal protocols. Operation of the device in multiple countries also requires multiple frequency band operation since communications frequencies are not commonly assigned in different countries.
Smaller packaging of state-of-the-art communications devices, such as personal communications handsets and laptop computers, does not provide sufficient space for the conventional quarter and half wavelength antenna elements. Physically smaller antennas operable in the frequency bands of interest (i.e., exhibiting multiple resonant frequencies and/or wide bandwidth to cover all operating frequencies of the communications device) and providing the other desired antenna-operating properties (input impedance, radiation pattern, signal polarizations, etc.) are especially sought after.
To overcome the antenna size limitations imposed by handset and personal communications devices, antenna designers have turned to the use of slow wave or meanderline structures where the structure's physical dimensions are not equal to the effective electrical dimensions. Recall that the effective antenna dimensions should be on the order of a half wavelength (or a quarter wavelength above a ground plane) to achieve the beneficial radiating and low loss properties discussed above. Generally, a slow-wave structure is defined as one in which the phase velocity of the traveling wave is less than the free space velocity of light. The wave velocity (c) is the product of the wavelength and the frequency and takes into account the material permittivity and permeability, i.e., c/((sqrt(εr)sqrt(μr))=λf. Since the frequency does not change during propagation through a slow wave structure, if the wave travels slower (i.e., the phase velocity is lower) than the speed of light, the wavelength within the structure is lower than the free space wavelength. The slow-wave structure de-couples the conventional relationship between physical length, resonant frequency and wavelength.
Since the phase velocity of a wave propagating in a slow-wave structure is less than the free space velocity of light, the effective electrical length of these structures is greater than the effective electrical length of a structure propagating a wave at the speed of light. The resulting resonant frequency for the slow-wave structure is correspondingly increased. Thus if two structures are to operate at the same resonant frequency, as a half-wave dipole, for instance, then the structure propagating a slow wave will be physically smaller than the structure propagating a wave at the speed of light. Such slow wave structures can be used as antenna elements or as antenna radiating structures.
Current antenna solutions for digital video broadcast (DVB) or digital television broadcast utilize external dongle antenna assemblies that are unwieldy, connected by wire to the television receiver and in most embodiments offer poor performance over a broad bandwidth. DVB systems may operate at the traditional television broadcast carrier frequencies, as well as cellular, PCS, DCS and UMTS carrier frequencies. Efficient antenna operation is desired over all operative frequency bands to permit a portable or mobile receiving device to receive multiple DVB signals. The use of multiple antennas within the receiving device is generally discouraged due to the space requirements for multiple antennas.
Reception of video signals by mobile or portable receivers is further complicated by signal fading and multi-path interferences. The problem of acceptable performance is exacerbated by the use of relatively simple receivers operating with a low gain antenna to receive the video signal.
Prior art television and video antennas include passive and active devices. Passive antennas may comprise a whip antenna or a loaded whip antenna having a length substantially less than ¼ wavelength at the operating frequency. Generally the whip (monopole) may exhibit fundamental resonance at one frequency somewhere in the desired spectral range covered by the receiver, with a maximum bandwidth limit governed by the well known Chu-Harrington relation. The Chu-Harrington limit establishes the minimum volumetric antenna size for a given bandwidth and radiometric efficiency; or conversely the maximum bandwidth the antenna will present for a given volumetric size and efficiency. At frequencies outside this bandwidth, the antenna becomes less efficient at converting received wave energy into a usable electrical signal. Nevertheless, whip antennas have been used for many years for portable television signal reception, albeit with non-optimal results.
An active solution for improving the bandwidth limitations of receive-only antennas is to incorporate an amplifier at the antenna terminals. The amplifier can be designed to match the impedance of the antenna over a broad frequency range, as is known. This approach has several drawbacks: 1) the amplifier must have a broad bandwidth and low noise contribution over the entire received signal frequency range, and 2) the amplifier must exhibit high linearity and low distortion even at high signal levels to prevent mixing of signals appearing in or out of band. With respect to item 1), the noise performance of the antenna amplifier combination is seldom as good as that achievable over a narrower bandwidth. Regarding item 2), proximity to high power transmitters widespread in urban environments can cause interference in even the best receiver designs. Also, signal mixing can produce spurious signals in the desired passband.
Very small antennas, as required in video-receiving laptop computers and handheld or portable video receivers, are particularly sensitive to noise interference from on-board digital circuits. This noise may be broadband or within the passband of the receiver's “front end” amplifier.